Nystagmus is a rapid involuntary oscillation of the eyeballs. Horizontal Gaze Nystagmus (HGN) refers to the lateral jerking of the eyes as they gaze to the side (e.g., a sudden jerking movement of the eyes in an opposing direction in relation to the direction at which the eyes are gazing). Vertical Gaze Nystagmus (VGN) refers to the vertical jerking of the eyes as they gaze vertically. HGN has been shown to be an effective indicator of alcohol impairment when administered by a trained law enforcement professional during field sobriety testing. HGN and/or VGN testing is administered because, when intoxicated, the brain experiences difficulty correctly controlling the eye muscles which presents as the nystagmus.
It has been demonstrated scientifically that there is a correlation between a Blood Alcohol Concentration (BAC) greater than a certain value (e.g., a BAC>0.10) and the presentation of HGN in an individual. HGN/VGN testing is used to assess the presence of “clues” in the visual tracking of a stimuli that point to intoxication in the test subject. Testing is typically conducted by a trained law enforcement professional by moving a stimuli (e.g., a pen or pen light) through the subject's field of view a number of times while looking for particular “clues” which present as a nystagmus. The professional is required to make a number of subjective determinations of the presence of nystagmus and point at which this nystagmus presents itself. Presentation of nystagmus during portions of the test at specific intervals and prior to a set point relative to geometry of the subject will indicate a failure of the test (e.g., BAC≧0.08), while no presented nystagmus will indicate a pass of the test (e.g., BAC<0.08).
Although the scientific nature, accuracy and reliability of HGN/VGN testing has long been established, there are several issues that affect the admissibility of the testing and results in a legal proceeding. The subjective nature of testing and pass/fail criteria is based upon the observations of a trained law enforcement professional rather than calibrated and tested equipment. A number of jurisdictions in the U.S. provide for the admission as evidence in a legal proceeding the testimony of a conducting law enforcement officer who has training and experience in conducting HGN/VGN and recognizing the characteristics of an intoxicated test subject. However, defense attorneys also often have an understanding of field sobriety testing such that, during a cross examination of the field test officer testifying as to the determination of an intoxicated defendant utilizing HGN/VGN testing, it might be possible to establish inconsistencies in how the test was administered or establish some level of doubt as to how the test results were interpreted by the field test officer. Further, due to the technical nature of the relationship between HGN/VGN testing and intoxication, some jurisdictions may further require expert witness testimony in relation to the testimony by a field officer who performed an HGN/VGN test to establish sobriety of the defendant.